We challenged 13 non asthmatics and 30 asthmatics with monosodium glutamate (MSG). Asthma was classified on the basis of clinical history and steroid use. MSG was administered as a graded single blind oral challenge followed by a placebo-controlled double-blind oral challenge in patients with pulmonary reactions during the single-blind challenge. Medications were withheld prior to study. MSG was administered in capsules containing increasing dosages of MSG at 30 min intervals for a total dose of 7.6 gm. A positive test was defined as a decrease in the FEV1 of greater than 20%. None of the non asthmatic subjects had any change in pulmonary parameters, and only 1 of 30 asthmatics had a significant drop in FEV1 (22%). This patient was entered into the double-blind challenge and no decrease in FEV1 was observed. We concluded that the ingestion of 7.6 gm of MSG over 2 hr or 6.0 gm at a single dose poses no consistent respiratory hazard to normal subjects and to the asthmatic population represented in this study. Interleukin-5 (IL-5), a lymphokine released by specific subsets of T cells following activation, is thought to be an important factor in eosinophil maturation and function. To determine whether local production of IL-5 by T cells in the mucosa could play a role in diseases characterized by mucosal eosinophilia, we have developed a sensitive and specific semiquantitative assay for IL-5 mRNA transcripts in mucosal biopsies. IL-5 mRNA was detected in gastric endoscopic biopsies from patients with eosinophilic gastroenteritis with multiple food allergies, but not in normal gastric mucosa. These results suggest T cell activation and production of IL-5 may play an important role in eosinophilic gastroenteritis.